1. Technical Field
Methods and example implementations described herein are generally directed to interconnect architecture, and more specifically, to Network on Chip (NoC) system interconnect architecture.
2. Related Art
The number of components on a chip is rapidly growing due to increasing levels of integration, system complexity and shrinking transistor geometry. Complex System-on-Chips (SoCs) may involve a variety of components e.g., processor cores, DSPs, hardware accelerators, memory and I/O, while Chip Multi-Processors (CMPs) may involve a large number of homogenous processor cores, memory and I/O subsystems. In both systems the on-chip interconnect plays a role in providing high-performance communication between the various components. Due to scalability limitations of traditional buses and crossbar based interconnects, Network-on-Chip (NoC) has emerged as a paradigm to interconnect a large number of components on the chip. NoC is a global shared communication infrastructure made up of several routing nodes interconnected with each other using point-to-point physical links.
Messages are injected by the source and are routed from the source node to the destination over multiple intermediate nodes and physical links. The destination node then ejects the message and provides the message to the destination. For the remainder of this application, the terms ‘components’, ‘blocks’, ‘hosts’ or ‘cores’ will be used interchangeably to refer to the various system components which are interconnected using a NoC. Terms ‘routers’ and ‘nodes’ will also be used interchangeably. Without loss of generalization, the system with multiple interconnected components will itself be referred to as a ‘multi-core system’.
There are several possible topologies in which the routers can connect to one another to create the system network. Bi-directional rings (as shown in FIG. 1(a)) and 2-D (two dimensional) mesh (as shown in FIG. 1(b)) are examples of topologies in the related art. Mesh can also be extended to 2.5-D (two and half dimensional) or 3-D (three dimensional) organizations.
Packets are message transport units for intercommunication between various components. Routing involves identifying a path composed of a set of routers and physical links of the network over which packets are sent from a source to a destination. Components are connected to one or multiple ports of one or multiple routers; with each such port having a unique ID. Packets carry the destination's router and port ID for use by the intermediate routers to route the packet to the destination component.
Examples of routing techniques include deterministic routing, which involves choosing the same path from A to B for every packet. This form of routing is independent from the state of the network and does not load balance across path diversities, which might exist in the underlying network. However, such deterministic routing may be simple to implement in hardware, maintains packet ordering and may be easy to render free of network level deadlocks. Shortest path routing may minimize the latency as such routing reduces the number of hops from the source to the destination. For this reason, the shortest path may also be the lowest power path for communication between the two components. Dimension-order routing is a form of deterministic shortest path routing in 2-D, 2.5-D, and 3-D mesh networks. In this routing scheme, messages are routed along each coordinates in a particular sequence until it reaches the final destination. For example in a 3-D mesh network, one may first route along the X dimension until it reaches a router whose X-coordinate is equal to the X-coordinate of the destination router. Next, the message takes a turn and is routed in along Y dimension and finally takes another turn and moves along the Z dimension until it reaches the final destination router. Dimension ordered routing is often minimal turn and shortest path routing.
FIG. 2 pictorially illustrates an example of XY routing in a two dimensional mesh. More specifically, FIG. 2 illustrates XY routing from node ‘34’ to node ‘00’. In the example of FIG. 2, each component is connected to only one port of one router. A packet is first routed over the x-axis till the packet reaches node ‘04’ where the x-coordinate of the node is the same as the x-coordinate of the destination node. The packet is next routed over the y-axis until the packet reaches the destination node.
In heterogeneous mesh topology in which one or more routers or one or more links are absent, dimension order routing may not be feasible between certain source and destination nodes, and alternative paths may have to be taken. The alternative paths may not be shortest or minimum turn.
Source routing and routing using tables are other routing options used in NoC. Adaptive routing can dynamically change the path taken between two points on the network based on the state of the network. This form of routing may be complex to analyze and implement.
A NoC interconnect may contain multiple physical networks. Over each physical network, there may exist multiple virtual networks, wherein different message types are transmitted over different virtual networks. In this case, at each physical link or channel, there are multiple virtual channels; each virtual channel may have dedicated buffers at both end points. In any given clock cycle, only one virtual channel can transmit data on the physical channel.
NoC interconnects often employ wormhole routing, wherein, a large message or packet is broken into small pieces known as flits (also referred to as flow control digits). The first flit is the header flit, which holds information about this packet's route and key message level info along with payload data and sets up the routing behavior for all subsequent flits associated with the message. Optionally, one or more body flits follows the head flit, containing the remaining payload of data. The final flit is the tail flit, which in addition to containing the last payload also performs some bookkeeping to close the connection for the message. In wormhole flow control, virtual channels are often implemented.
The physical channels are time sliced into a number of independent logical channels called virtual channels (VCs). VCs provide multiple independent paths to route packets, however they are time-multiplexed on the physical channels. A virtual channel holds the state needed to coordinate the handling of the flits of a packet over a channel. At a minimum, this state identifies the output channel of the current node for the next hop of the route and the state of the virtual channel (idle, waiting for resources, or active). The virtual channel may also include pointers to the flits of the packet that are buffered on the current node and the number of flit buffers available on the next node.
The term “wormhole” plays on the way messages are transmitted over the channels: the output port at the next router can be so short that received data can be translated in the head flit before the full message arrives. This allows the router to quickly set up the route upon arrival of the head flit and then opt out from the rest of the conversation. Since a message is transmitted flit by flit, the message may occupy several flit buffers along its path at different routers, creating a worm-like image.
Based upon the traffic between various end points, and the routes and physical networks that are used for various messages, different physical channels of the NoC interconnect may experience different levels of load and congestion. During congestion, when multiple sources transmit messages to the same destination, their messages may contend with each other and with the cross-traffic for the bandwidth. Therefore, the effective destination bandwidth received by each source will depend on their positions in the network, how their routes overlap with each other, cross-traffic along their routes to the destination, and the arbitration policies deployed at various routers where arbitration is needed. In spite of uniformly fair arbitration policies at all routers, depending on location of various sources there may be a substantial difference in the destination bandwidth received.
Consider a section of a NoC interconnect shown in FIG. 3, wherein four components (source 1, source 2, source 3, and source 4) transmit messages to one component (destination). In this example, the maximum data transmit bandwidth of the four source components is equal to the maximum data receive bandwidth of the destination component. Each of the five components are connected to a local router node, and the router nodes are connected with each other using point to point channels as shown in FIG. 3. In the example of FIG. 3, each of the channels have a receive bandwidth of the destination component equal to a transmit bandwidth of the source component.
In the system shown in FIG. 3, if all four source components attempt to transmit data at their peak transmit rate and if the destination component is ready to accept data at its peak receive rate, then messages from the four source components will contend with each other within the NoC interconnect.
In FIG. 4, the routers and components are separated for clarity the channels that connect components with their local routers are illustrated.
At router 41 in FIG. 4, messages arriving at the left input port (e.g., from router 42) and the bottom input port (e.g., from source 4) will contend for the right output port (e.g., to router 40). If routers implement uniformly fair arbitration policy to arbitrate between incoming messages at different input ports contending for an output port, then the output port's bandwidth will be equally split between the two input ports as shown. Each input port will receive 50% of the destination bandwidth—source 4 therefore will receive half of the destination bandwidth.
At router 42 in FIG. 4, messages arriving at the left input port (e.g. from router 43) and bottom input port (e.g., from source 3) will contend for the right output port. If routers implement uniformly fair arbitration policy to arbitrate between incoming messages at different input ports contending for an output port, then the 50% output port's bandwidth (computed in the above step) will be equally split between the two input ports as shown. Each input port will receive 25% of the destination bandwidth—source 3 therefore will receive a quarter of the destination bandwidth.
At router 43 in FIG. 4, messages arriving at the left input port (e.g., from router 44) and bottom input port (e.g., from source 2) will contend for the right output port (e.g., to router 42). If routers implement uniformly fair arbitration policy to arbitrate between incoming messages at different input ports contending for an output port, then the 25% output port's bandwidth (computed in the above step) will be equally split between the two input ports as shown. Each input port will receive 12.5% of the destination bandwidth—source 2 therefore will receive 12.5% of the destination bandwidth. The remaining 12.5% bandwidth will be received by source 1.
The example of FIG. 4 illustrates that even though each router employs a uniformly fair arbitration policy wherein the router gives fair share of output port bandwidth among all input port contenders, the four sources receive vastly different shares of the destination bandwidth. In a complex network with additional cross-traffic, the bandwidth allocated to various source components when they content for various destinations may vary substantially. This may be undesirable in many applications, wherein fair or equal allocation of various resources among all contenders may be important to achieve a high system performance. In many systems, weighted allocation is desired, so that the various resource bandwidths are allocated among various contenders in a pre-specified ratio.
There are several techniques in the related art to provide uniform or weighted fair arbitration within a single router, wherein the output port bandwidth is allocated to contending input ports based on the weight specification. Weighted round-robin, deficit round-robin, weighted fair queuing, etc. are a few techniques that are used in the related art. Guaranteeing weighted- or uniform-allocation of various resources among contenders in a distributed NoC interconnect with resources and contenders connected at arbitrary positions in the NoC interconnect is challenging. A few techniques that are used in the related art are described below.
Rate limiting the sources: Each source contending for a resource destination is allowed to send data at a pre-specified rate based on its fair share. This technique is independent of the state of other sources, whether the other sources are contending for the resource or not. Therefore, based upon the pre-specified rates of sources, rate limiting of the sources can either lead to under-utilization of resource bandwidth, or unfair allocation.
Configure router port weights: Based on the traffic merging characteristics, various ports of the routers along the path from various sources to the resource destination are assigned a weight. The weight is used for local arbitration at the router. If all sources are participating, then the configuration of router port weights can provide fair allocation. However when several sources are not contending for the resource, then unfair allocation may occur.
Age based arbitration: Every message injected by various components carries timestamp information, which describes the age of the message. Within the NoC interconnect, routers give higher preference to older messages over newer messages, whenever multiple messages content for an output port. This technique can provide end-to-end uniform fairness, however it is unable to provide weighted fairness. Furthermore, age based arbitration comes at a high implementation cost of additional bits needed to carry the age information and complex circuitry at every router to determine the oldest message.